Is there a punishment in Islam for those who beat their wives without any valid reason? Even-though the wife is so obedient and take care of the home and the children?
First off, a women is a very precious person in the view of her husband, her sons, her brothers, and her father. The third longest surah in the Quran (Al Nisa) is dedicated to the the woman.
Although your question raises the issue of a punishment for a man that beats his wife, let me just address the issue of a man that might, by the slightest of slight touches, hit his wife. Now, let me readdress, this really concerns a very slight hit, but it also is a last resort for a man that might hit his wife:
“As to those women on whose part you fear ill-conduct, admonish them (first), (next), refuse to share their beds, (and last) beat them (lightly, if it is useful), but if they return to obedience, seek not against them means (of annoyance). Surely, Allaah is Ever Most High, Most Great.” [al-Nisaa’ 4:34]
As I mentioned, it is the third resort after admonishing them, and then refusing to share their beds that is slightly hitting one's wife.
However, your question addresses beating, which in my view, is not by any means lightly hitting. As I mentioned, a woman should be treated with the utmost respect in Islam:
If a woman rebels against her husband and disobeys his commands, then he should follow this method of admonishing her, forsaking her in bed and hitting her. Hitting is subject to the condition that it should not be harsh or cause injury. Al-Hasan al-Basri said: this means that it should not cause pain.
‘Ata’ said: I said to Ibn ‘Abbaas, what is the kind of hitting that is not harsh? He said, Hitting with a siwaak and the like. [A siwaak is a small stick or twig used for cleaning the teeth - Translator]
There are countless other ayat in the Quran that demonstrate the above points (especially in the surah Al Nisa). I urge you to take a look at this site that gives a few good examples of ayat in the Quran on the subject.
In any case, the beating of the wife is clearly not permissible in Islam. It remains a sin. Whether there is a punishment in the world or in the afterlife remains a choice of Allah. He is the All Knowing.
There is no specific punishment. However, In the same verse that mentions beating, Allah sends a really strong warning to those who abuse this command:
Men are in charge of women by [right of] what Allah has given one over the other and what they spend [for maintenance] from their wealth. So righteous women are devoutly obedient, guarding in [the husband's] absence what Allah would have them guard. But those [wives] from whom you fear arrogance - [first] advise them; [then if they persist], forsake them in bed; and [finally], strike them. But if they obey you [once more], seek no means against them. Indeed, Allah is ever Exalted and Grand. http://quran.com/4/34
According to the tafsir, the last part of the verse reminds muslims that Allah ever exalted and Grand, In other words, if you abuse this verse to beat a poor women, then there is no one as Grand and Exalted as Allah who can give you a much severed punishment than what you did to your wife. So fear him and don't abuse this order which came to solve issues, not to injure your wife(ives).
If you want to know more about this verse or the conditions which allow you to beat your wife check this question: Does the Quran allow husbands punish their wives?